1. Field of the Invention
The present invention relates to a method suitable for forming a deposited film, particularly a functional film, and more particularly an amorphous or crystalline deposited film adapted for use in semiconductor devices, electrophotographic photosensitive devices, image reading line sensors, image pickup devices, photoelectric elements or the like.
2. Related Background Art
As an example, for formation an amorphous silicpn film, various methods have been tried methods such as vacuum deposition, plasma CVD, CVD, the reactive sputtering, ion plating, photochemical vapor deposition etc. Generally, the plasma CVD method has been widely used and industrialized.
However, such deposited film of amorphous silicon still needs to be improved in their electrical and optical properties, fatigue characteristic due to repeated use, resistance to environmental conditions, productivity, and mass producibility including uniformity and reproducibility.
The reaction process in the plasma CVD process commonly used up to now in the formation of an amorphous silicon film is considerably more complex in comparison with that of the conventional CVD process, and has not fully been analyzed. Also the deposited film formation involves various parameters such as the substrate temperature, flow rates and ratio of introduced gases, pressure during film deposition, high-frequency electric power, structure of electrodes, structure of reactor vessel, rate of gas discharge, method of plasma formation etc. of which adjusting a combination thereof often results in an unstable plasma or undesirable effects on the formed film. Besides, since each apparatus has its own specific parameters, it is difficult to generalize the production conditions.
On the other hand, the plasma CVD method is presently considered best for forming an amorphous silicon film having electrical and optical properties for various applications.
However, for certain applications requiring reproducible mass production of a large area with uniform thickness and quality, the plasma CVD method requires a significant investment in equipment. Use of such equipment is inevitably concerned with complex items of control, narrower tolerances and delicate adjustments. For this reason improvements on these points have been longed for.
On the other hand, the conventional CVD process requires a high temperature, and is unable to produce a deposited film having substantially acceptable properties.
In this manner, there has been desired a process for forming an amorphous silicon film with an inexpensive apparatus while maintaining substantially acceptable properties and uniformity.
For avoiding such drawbacks of the plasma CVD process, there is already proposed a novel process of activating a material for film formation in a separate "activating space" to obtain an active species, and introducing said active species alone into a film forming space for effecting film formation.
However, since such process generally employs a commercially available and relatively stable lower order silane compound such as SiF.sub.4 or Si.sub.2 F.sub.6 as the material for producing a precursor for film formation in the activating space, such process requires a relatively large excitation energy such as electrical energy obtained for example by microwave, high-frequency current or DC current, thermal energy obtained for example by resistor heating or high-frequency heating or optical energy. Consequently, in this process it is difficult to improve the efficiency of activation beyond a certain point, as well as requiring further improvements in terms of the amount of energy required and the efficiency of consumption of the raw material gas, in order to achieve mass production with a low cost in a simple apparatus.